Manometer is a very simple device use to measure a pressure. In manometer U shape glass tube fill with water or mercury . When gas pressure is applied proportionately hight of fill fluid is shifted and we can measure pressure. That is why pressure is often measured in unit of hight like inches of water, inches of mercury...etc
Manometer come in a variety of forms. Most common being the U-tube manometer, Raised well manometer, inclined manometer. Venting one side of manometer is a standar practice for measuring a guage pressure. Both pressure port will be use if we want measure a differential pressure. Absolute pressure may also be measured by manometer if one of the pressurre port is connected to seal vaccume chamber.